The present invention relates to radio systems, and in particular to the allocation of bandwidth within a radio system. More especially but not exclusively it relates to cellular mobile radio systems.
Cellular mobile radio systems include means for optimising the number of communications channels or connections which can be supported within an allocated radio spectrum. This is achieved in Time Division Multiple Access (TDMA) systems by dividing the two orthogonal dimensions of time and frequency, to give a discrete number of communication channels. This allows a number of mobiles to access a given segment of radio frequency spectrum contemporaneously.
With Code Division Multiple Access (CDMA) systems, communication is effected by means of three orthogonal dimensions, which are time, frequency and code. Unlike TDMA systems which only use the dimensions of time and frequency, CDMA systems offer a third orthogonal dimension, that of code. Each mobile is allocated a separate code which is used in transmission to modulate a data signal, thereby spreading the bandwidth of a subsequently transmitted radio signal within a shared radio spectrum. At a base station a receiver de-spreads the received radio signal using this known code, to recover a mobile's data. All other coded signals appear as noise. However, as the number of mobiles in a CDMA system increases, the noise level which is present when de-spreading a desired signal increases. This, by its nature, increases the symbol error rate in the integrity of the received data; an effect which is experienced by all mobiles. Thus in CDMA systems there is not a hard capacity limit as there is in TDMA systems, but a soft limit, derived from an acceptable symbol error rate which a mobile user can endure. Nevertheless, for a predetermined bandwidth and an acceptable symbol error rate, there is an upper limit in the capacity of a CDMA system as determined by the three dimensions of time, frequency and a predetermined number of codes which are shared between all mobiles. For TDMA systems, an upper limit in the capacity of a system is fixed by the number of frequency channels and the number of time slots those channels are divided into.
In the following description we define effective bandwidth to mean a collection of quantitative parameters that determine an information bearing capacity or resource of that system. Thus for a CDMA cellular radio system effective bandwidth would mean an aggregate of the information bearing resources of the system, taking account of a pre-determined number of codes, number of carrier frequencies, and number of time slots of that system. The total effective system bandwidth is therefore a total of all of these quantitative parameters which are available for the support of data services.
In the future, radio communications systems will be expected to support a range of different services such as voice, video and various types of data services. Some services are rather `bursty` in nature in that they are characterised as having a high peak to mean data rate ratio. This results in there being periods of high activity followed by periods of low or zero activity during a call.
The radio spectrum is a precious resource. One way of ensuring optimum use of an available effective bandwidth resource is, where possible, to take advantage of the `bursty` nature of a mobile's data source so that bandwidth resource is only occupied when the source is active, or in other words, when it has data to send. In this way, instead of an available bandwidth resource being occupied or allocated on a basis of a peak data rate predicted for a mobile's data source, this bandwidth is occupied or allocated on a basis which is closer to the mean data rate of a mobile's data source. This provides for a greater utilisation of the available effective bandwidth resources.
In a mobile radio system, data is usually transported between a transmitter and a receiver in quantised units. This is a feature which is embodied in the structure of the transport chain of a radio system. Since data is transported in data units of a known and predetermined size, an effective throughput of a radio connection can be determined from a number of data units transported in a pre-determined time interval.
Typically data connections can accept delays of hundreds of milliseconds or more. Delay occurs when a combined total data rate as demanded by all active data connections exceeds a data rate which can be supported by a radio system, as determined by an available bandwidth resource. In this case, a resource allocation scheme is required to fairly allocate an effective bandwidth resource to the mobiles of a radio system, so as to optimise fairly the throughput of a mobile's data source, whilst minimising delay suffered by that source.